The long-range objective of this project is to understand the functional organization of primate retina. Our present experiments on the rhesus monkey are concerned with: (1) identifying the ultrastructural characteristics of different types of photoreceptors and of photoreceptors in different areas of retina which may correlate with the electrophysiological responses of these cells; (2) examining light/dark differences in activity of photoreceptor terminals with the electron microscope, as measured by uptake of the enzyme tracer horseradish peroxidase into synaptic vesicles; (3) characterizing light/dark ultrastructural changes within photoreceptor terminals; (4) identifying specific photoreceptor systems using horseradish peroxidase uptake into synaptic vesicles and selective chromatic adaptation.